NATURE OF THE SI-N BOND IN SILATRANES

Both ab initio and semiempirical calculations are used to investigate the structure and bonding in substituted silatranes. In agreement with gas-phase experiments, which find Si-N distances that are 0.25-0.35 angstrom longer than those in the crystal phase, the Si-N distance (for example, in methylsilatrane) is predicted to be much larger than that observed in the crystal. Nonetheless, a bond critical point is found between Si and N, suggesting the existence of an Si-N bond in these compounds. It is found that the energy cost for constraining the Si-N distance in hydroxysilatrane to a value similar to that observed in the crystal is less than 6 kcal/mol. This suggests that crystal forces may be responsible for the much shorter Si-N distance in the solid.